[Meffy]

Sounds like too much humidity indoors to me. I've got to install a ceiling fan in our main bathroom to vent the saturated air from showers outdoors so we don't have the same problem. (I suspect that's our main source of overly humid air.)

[edit] I'm no expert, this is an educated guess merely. But it seems a safe bet it's fairly close to the mark. What's causing the excess humidity is another matter altogether.